The present invention relates to waveguide switching arrangements for coupling microwave energy to an external load.
As will be described, the present invention essentially is an improvement of a microwave switching concept disclosed in a co-pending patent application, Ser. No. 928,218 "Pulse Generator Utilizing Superconducting Apparatus" filed by the present inventor July 26, 1978, and now U.S. Pat. No. 4,227,153. A publication "Microwave Power Gain Utilizing Superconducting Resonant Energy Storage", Applied Physics Letters (Appl. Phy. Lett. 32(1), January 1, 1978 also discloses this concept.
In these prior disclosures a rectangular waveguide is coupled to a resonator and a low power microwave single source is provided with a lateral branch or T having an H-plane junction with the main cavity of the guide. A short circuit formed at the terminal end of the cavity reflects incident microwave radiations in a standing wave interference pattern and the relative spacing of the short and the T-opening is such that the interference pattern produces a node (point of minimum electrical field density) at the input to the T. The output of the T terminates in a matched load, such as an antenna, which due to the node, is isolated from the microwave energy in the main cavity during its normal operation or, in other words, its `open` state. To `close` the switch, the short circuit essentially is displaced toward the T by a quarter wavelength of the microwave signal. The displacement then produces a maximum field intensity (anti-node) at the T to couple the load and release the microwave energy in the form of a high-power pulse.
In its preferred form, the present invention utilizes a configuration similar to that just described. The single fundamental difference involves the manner in which the short circuit is displaced. Thus, in the earlier configuration, a gas discharge tube was physically mounted in the rectangular waveguide cavity at the desired quarter wavelength spacing from the terminal, short-circuit end of the guide. By applying a suitable DC voltage across internal electrodes of the discharge tube, a contained gas, such as Xenon, ionizes to initiate the reflection and produce the antinode which releases the energy to the load.
Although the use of the gas discharge tube is effective and beneficial, there are certain inherent deficiencies. For example, the plasma discharge tube necessarily is positioned in the region of electrical field intensity and, consequently, residial losses associated with it materially limit switch isolation. Thus, in the open position of a switch using superconducting components, an isolation of about 70 db is obtained at s-band. With the discharge tube removed, the s-band experiment has an isolation in excess of 120 db. To a lesser extent, the residual losses are associated with the tuning dielectric. Further, in some applications, both the rise time and the duration time of the output pulse are matters of critical concern. As to rise time, it is affected by the time needed for the gas of a discharge tube to ionize and produce the plasma. With regard to duration time, once the gas is ionized it requires a certain time for its recombination. During this recombination period, it continues to screen microwave reflection.
In the present invention, instead of using an ionizable gas, such as the gas fill of the discharge tube, switching is achieved by generating a beam of electrons directed traversely of the wave guide cavity in a direction parallel to its electrical field. As will be understood, the beam must be sufficiently intense, i.e. sufficient electron density, to produce the reflection and the desired standing wave form. In one form of the invention, the electron beam is generated externally of the waveguide so as to penetrate its wall and traverse the central portion. In another form, a vacuum arc of pure electrons is produced by applying the high voltage to an electrode provided in the waveguide wall.